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10 & 100 More or Less: Mental Math!

Second graders learn to add and subtract 10 and 100 in their heads — by changing just one digit — a mental-math superpower built on understanding place value.

Grade 2Mental Math50 minutes1 class periodGradual ReleaseExplicit teaching4 StandardsCommon Core
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Lesson at a Glance

Everything you need before the bell rings

Learning Objectives

Students will be able to…

  • ✓Add and subtract 10 mentally.
  • ✓Add and subtract 100 mentally.
  • ✓Change the right digit.
  • ✓Explain the place-value pattern.
Essential Question

What is 234 + 100? You can answer in your head in one second — without adding it out! What is the secret?

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Lesson Phases
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Vocabulary Terms
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Standards Aligned
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Interactive Task
Mental Math Explorer · Interactive

The Mental Math Explorer

Project this and tap each idea. Have students see how adding 10 or 100 changes just one digit — mental math made easy!

🧠 Explore 10 & 100 more or less — tap eachTry it

The Lesson · Gradual Release (I Do · We Do · You Do)

50 minutes, five moves

Tap any phase to open the teacher moves and student actions.

1

Launch — Fast in Your Head

5 min

What is 234 + 100? Can you answer without adding it all out?

👩‍🏫 Teacher Moves

  • Ask 234 + 100.
  • Ask how they knew so fast.
  • Set the goal: mental math.

🎒 Student Actions

  • Try the problem.
  • Wonder how.
  • Predict the trick.
2

I Do — Change One Digit

10 min

Teacher models.

👩‍🏫 Teacher Moves

  • +10 changes the tens.
  • +100 changes the hundreds.
  • The other digits stay the same.

🎒 Student Actions

  • See +10.
  • See +100.
  • Note what stays.
3

We Do — Explore Together

13 min

Class uses the Mental Math Explorer.

👩‍🏫 Teacher Moves

  • Tap each idea.
  • Do 10 more/less mentally.
  • Do 100 more/less mentally.

🎒 Student Actions

  • Tap each idea.
  • Try 10 more/less.
  • Try 100 more/less.
4

You Do — On Your Own

12 min

Students do mental math.

👩‍🏫 Teacher Moves

  • Add and subtract 10 and 100.
  • Do them in their heads.
  • Explain which digit changed.

🎒 Student Actions

  • Do the math.
  • Do it mentally.
  • Explain it.
5

Close — Close

5 min

One mental-math problem.

👩‍🏫 Teacher Moves

  • Give a 10 or 100 more/less.
  • Answer mentally.
  • Hand out the exit ticket.

🎒 Student Actions

  • Answer it.
  • Explain it.
  • Complete the exit ticket.
Standards Alignment

Built to the standards you report on

Aligned to the Common Core State Standards for Mathematics.

CCSS
2.NBT.8

Mentally add 10 or 100 to a number, and subtract 10 or 100, within 900.

CCSS
2.NBT.1

Understand three-digit place value.

CCSS
2.NBT.5

Fluently add and subtract within 100.

SMP
MP.7

Use place-value structure to compute mentally.

Differentiation

One lesson, every learner

Multilingual Learners

ELL / EMERGING READERS
  • Use a place-value chart.
  • Sentence frame: “Ten more changes the ___ digit.”
  • Show with base-ten blocks.

Support & Access

IEP / 504
  • Start with 10 more only.
  • Use a hundred chart.
  • Point to the digit that changes.

Stretch & Extend

GIFTED / EARLY FINISHERS
  • Do 10/100 more and less in a chain.
  • Explain a tricky one (with regrouping).
  • Find a mystery number.
Materials

What to gather

  • 📽️Projector / board
  • 🔢Hundred charts
  • 💻The Mental Math Explorer
  • 🧮Base-ten blocks
  • ✏️Pencils
  • 🎫Exit-ticket slips
Vocabulary

Key terms — hover for a quick definition

mental mathmath you do in your headmorea greater amountlessa smaller amounttens digitthe digit in the tens placehundreds digitthe digit in the hundreds placeplace valuewhat a digit is worth by its placepatterna repeating ruledigita single number symbol
Evaluate

Exit Ticket

Preview the three formative checks. Tap “Sample answer” to see what mastery looks like — hide them before you print for students.

QUESTION 1
What is 56 + 10?
66 (the tens digit changes; the ones stay the same).
QUESTION 2
What is 428 − 100?
328 (only the hundreds digit changes).
QUESTION 3
When you add 10 to a number, which digit changes?
The tens digit.

Going deeper? Mental-math chains.

Have students do a chain in their heads — like 200, +100, +10, −10 — saying each answer aloud. A printable mental-math sheet is in the Math library.

Study · Flashcards

Study the key terms

Tap a card to flip it, then rate whether you knew it. Built from this lesson’s vocabulary.

🃏 10 & 100 More or LessFlip
Card 1
Term
Tap to flip →
Meaning
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Nice work!

Practice · Quiz

Check your understanding

A quick self-check with instant feedback, drawn from this lesson’s key terms.

📝 10 & 100 More or LessQuiz
Score: 0
1 / 6
Question 1
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Practice · Worksheet

Printable worksheet

A print-and-go review sheet with a built-in answer key. Tap “Show answer key” to reveal answers, or print the clean version for students.

🖨️ 10 & 100 More or LessPrint
Name: ________________________
Date: ____________

Part A · Write the word that matches each meaning

Word bank: digit, hundreds digit, less, mental math, more, pattern, place value, tens digit
  1. the digit in the tens place
  2. a greater amount
  3. a single number symbol
  4. the digit in the hundreds place
  5. math you do in your head
  6. what a digit is worth by its place
  7. a smaller amount
  8. a repeating rule

Part B · Show what you learned

  1. What is 56 + 10?
  2. What is 428 − 100?
  3. When you add 10 to a number, which digit changes?
Answer key — Part A: 1) tens digit · 2) more · 3) digit · 4) hundreds digit · 5) mental math · 6) place value · 7) less · 8) pattern
Part B: 1) 66 (the tens digit changes; the ones stay the same). 2) 328 (only the hundreds digit changes). 3) The tens digit.